The invention is a method for the treatment of cancers such as non-Hodgkin""s lymphomas (NHLs). The method utilizes shiga toxin or shiga-like toxin-1 to selectively kill cancer cells in bone marrow or peripheral blood ex vivo prior to reinfusion. The elimination of cancer cells by the method of the invention may provide a cure for some cancers.
Shiga-like toxin-1 (SLT-1) is a bacterial toxin that, along with shiga toxin itself, binds to CD77, a cell surface glycolipid, and kills cells by inhibiting protein synthesis1,2. In the human hematopoietic system, CD77 expression is restricted to a subset of activated B cells3-8.
Twenty thousand North Americans died of non-Hodgkin""s lymphomas in 1994 alone. A large proportion of NHLs are follicular (B cell) lymphomas which are classified as low-grade lymphomas and for which no curative treatment exists9. Autologous bone marrow transplantation (ABMT) in patients with B cell malignancy is increasingly used as a therapeutic option. The transplanted marrow is frequently contaminated with residual cancer cells, which ultimately leads to a relapse of the patient. This contamination is a particular problem in relation to the treatment of follicular lymphomas; therefore, a safe and effective way of killing cancers cells, while sparing hematopoietic stem cells, is required for ABMT to become a useful and front-line therapy. Bacterial, plant and fungal toxins represents some of the most potent cytotoxic agents known; however, their toxicity cannot be exploited until such molecules can be targeted to specific cancer cells. A small subset of toxins in the context of an immunotoxin (antibody conjugate) or a fusion protein have been used in phase I or phase II trials in humans10,11. These toxin conjugates have met with limited success12,13.
The invention provides a method for the ex vivo purging of bone marrow or peripheral blood containing stem cells prior to transplant using shiga toxin or shiga-like toxin-1 which binds specifically to the cell surface glycolipid CD77, so that all CD77-expressing cells in the bone marrow or blood sample are killed while the normal hematopoietic precursor cells are spared. The binding specificity of the toxin to the CD77 receptor can also be used to selectively remove CD77+ cells from bone marrow or blood samples using affinity chromatography.
The present invention preferably utilizes an unconjugated native bacterial toxin, shiga-like toxin-1 (SLT-1), as a chemotherapeutic drug in the ex vivo purging. For the purposes of the present invention SLT-1 functions equivalently to shiga toxin itself. SLT-1 is preferred because the expression system for this toxin is readily available26. Ex vivo purging avoids complications relating to the toxin""s systemic toxicity. Severe combined immunodeficient (SCID) mice were used as recipients for SCID bone marrow seeded with the human Burkitt""s lymphoma cell line, Daudi, as a model system for SLT-1 purging of B-cell NHL ex vivo. The SCID/Daudi model system has been well studied for in vivo experiments of B-cell immunotoxins14,15.